1. Field of the Invention
This invention relates to a dual-sided heat dissipating structure and its fabrication method for integrated circuit package and particularly to Ball Grid Array package (BGA package) that has two lateral sides engageable respectively with a heat dissipating member for enhancing heat dissipating effect.
2. Description of the Prior Art
In semiconductor integrated circuit (IC) industry nowadays, how to make IC packages small size with good heat dissipating property at low cost is a goal almost universally pursued. Heat dissipating property of IC package directly affects IC performance and reliability. It is a subject attracted heavy research and development.
One of the techniques being used to address this issue is generally called "Enhanced BGA" (EBGA). FIG. 1 illustrates an example using EBGA. The EBGA package 10 includes an IC chip 11, a Tape Automated Bond Tape (TAB Tape) 12 which has a metallic circuitry 121 formed thereon, a plurality of solder balls 13 and a metallic heat dissipating member 16. The solder balls 13 are attached to a bottom side of the TAB Tape 12 and are nested in a layer of non-conductive solder ball mask 14 for preventing short circuit of the circuitry 121 resulting from oversize solder balls 13. The TAB Tape 12 has a center opening to accommodate the IC chip 11. The IC chip 11 couples with the circuitry 121 by means of gold wires 17. As the TAB Tape 12 is thinner than conventional wiring frame and is more desirable for multilayer circuit design, high performance and complex and multilayer metallic circuitry 121 may be produced. The whole EBGA package 10 may be made thin and small size. Furthermore the IC chip 11 and the TAB Tape 12 are adhered to the heat dissipating member 16 by means of EPOXY 18 which has good heat dissipating property. The heat dissipating member 16 also provides support for the IC chip 11 and the generally soft TAB Tape 12.
However the structure set forth above has a non-active side of the IC chip 11 adhering to the heat dissipating member 16. The active side i.e., the side where semiconductor circuitry located of the IC chip 11 is covered by a layer of underfilling 15 through molding process. The under filling 15 is usually a poor heat conducting resin. Most heat generated in the IC chip is taking place on the active side. The heat dissipating member 16 attached to the non-active side of the IC chip 11 thus has only limited effect on heat dissipation of the IC chip.
FIG. 2 illustrates another conventional Tape BGA package structure. The TBGA package 20, like the EBGA package shown in FIG. 1, also has an IC chip 21, a TAB Tape 22, a plurality of solder balls 23, a heat dissipating member 26 and a bottom underfilling 25. However in the TBGA package 20 the metallic circuitry forms a plurality of inner leads 222 extending to a center opening of the TAB Tape 22. The inner leads 222 are taping to bonding pads 211 located on the IC chip 21 by means of Tape Automated Bonding (TAB) process. Wire bonding process used in the EBGA package is thus omitted. Production time and cost are lower. But there is no much improvement in heat dissipating effect since the heat dissipating member 26 is still attached on the non-active side of the chip 21.
FIG. 3 illustrates yet another example of EBGA package 30 which uses a printer-circuit-board-like substrate 32 to support an IC chip 31. There are inner solder balls 37 to couple the active side of IC chip 31 with the metallic circuitry 321 laid on the substrate 32. An underfilling 35 is poured between the IC chip and the substrate and surrounds the inner solder balls 37 to protect the circuitry for enhancing the package reliability. A dish-shaped heat dissipating member 36 adheres to a non-active side of the IC chip 31 and the substrate 32 by means of EPOXY adhesive 38 (or a heat conductive resin) which has relative good heat conductivity. Like the previous examples, this structure also is not able to improve heat dissipating function very much. Moreover there is a closed void space 39 formed between the heat dissipating member 36 and the IC chip 31 and the substrate 32. The close void space 39 tends to trap moisture either from atmosphere or EPOXY adhesive 38. In the manufacturing process, when the EBGA package 30 is soldered to a printed circuit board or under IR Reflow test, it will be heated to 230.degree. C. in a short time. The moisture trapped in the void space 39 will be vaporized and expanded and results in the heat dissipating member 36 breaking away from the substrate 32 (commonly called "Pop Corn Effect"). In order to prevent the Pop Corn Effect from happening, an additional procedure of vacuuming or filling nitrogen gas in the void space 39 is necessary. This additional procedure will increase production complexity and cost and still cannot totally eliminate the Pop Corn Effect.
Then there is a further EBGA package scheme being introduced as shown in FIG. 4. The EBGA package 40 has a heat dissipating member 46 adhering to an active side (i.e., where gold lines 47 are bonded) of the IC chip 41. While the IC chip 41 generating heat may be dissipated quickly through the heat dissipating member 46, the non-active side of the chip 41 adheres to the substrate 42 has relatively poor heat dissipating effect. Moreover to protect the gold wire 47 from unduly contact with the surroundings, the heat dissipating member 46 should be made in a T-shape and should have a greater thickness. This will make the whole package bulky.
All of this shows that there is still a lot of room for improvement regard producing low cost IC package at small size with high heat dissipating efficiency.